High-energy X-ray-spectroscopy-based inspection system and methods to determine the atomic number of materials

ABSTRACT

The application discloses systems and methods for X-ray scanning for identifying material composition of an object being scanned. The system includes at least one X-ray source for projecting an X-ray beam on the object, where at least a portion of the projected X-ray beam is transmitted through the object, and an array of detectors for measuring energy spectra of the transmitted X-rays. The measured energy spectra are used to determine atomic number of the object for identifying the material composition of the object. The X-ray scanning system may also have an array of collimated high energy backscattered X-ray detectors for measuring the energy spectrum of X-rays scattered by the object at an angle greater than 90 degrees, where the measured energy spectrum is used in conjunction with the transmission energy spectrum to determine atomic numbers of the object for identifying the material composition of the object.

CROSS-REFERENCE

The present application is a continuation of U.S. patent applicationSer. No. 13/033,590, filed on Feb. 23, 2011, now U.S. Pat. No.8,750,454, issued on Jun. 10, 2014, which, in turn, relies on U.S.Patent Provisional Application No. 61/308,152, filed on Feb. 25, 2010,for priority. All priority applications are hereby incorporated byreference in their entirety.

FIELD OF THE INVENTION

The present specification generally relates to the field of radiantenergy imaging systems, and more specifically to a system that makes useof the full spectrum of X-ray energies transmitted or transmitted andscattered for improved determination of the atomic number of materials,such as objects in cargo containers.

BACKGROUND OF THE INVENTION

X-ray inspection systems are presently limited in their ability todetect and distinguish contraband, drugs, weapons, explosives, and otheritems of interest concealed in cargo from benign materials. There isfurther an interest in inspecting cargo for manifest verificationpurposes to ensure appropriate customs duty is paid.

The intensity of the transmitted X-rays is related to the areal density(i.e. density×thickness) and the atomic number (Z) of the materials theytraverse. Radiographs produced by conventional X-ray systems are oftendifficult to interpret, because objects are superimposed and no Zinformation is provided. Therefore, a trained operator must study andinterpret each image to render an opinion on whether or not a target ofinterest, or a threat, is present. When a large number of suchradiographs are to be interpreted, such as at high-traffic transitpoints and ports, these inherent difficulties, combined withoperator/screener fatigue and distraction, can compromise detectionperformance. There is a need for automatic detection and/orscreener-assist tools for detection of threats and other targets, inorder to improve the efficiency and accuracy of operators, and to reducethe number of operators needed for the detection.

Methods known to those skilled in the art for obtaining usefulZ-information include the use of dual-energy X-ray sources, anddual-species technologies (X-ray inspection combined with neutroninspection). However, these methods do not readily allow accuratedetermination of the actual Z of the cargo contents, but rather yieldsan average Z that represents a mix of the various materials in the X-raybeam path. Thus, these methods are not efficient.

Therefore, X-ray inspection systems currently available in the artprovide limited accuracy for detection of items of interest. Further,these systems do not effectively detect high atomic-number (“high-Z”)materials. Detecting such materials, particularly smuggled specialnuclear materials (SNM) that could potentially be used to make a weapon,or materials used to shield their radioactive emissions, is a verycomplex task. One of the materials of greatest concern, highly enricheduranium (HEU), has a relatively low level of radioactivity. Plutonium,another nuclear weapons grade material, has a higher specific activityand higher-energy emissions. However, it can be shielded by employing acombination of high-Z materials for shielding gamma rays and low-atomicnumber (“low-Z”) neutron absorbers for shielding neutrons produced byspontaneous fission. Thus, it is very difficult to detect shielded orconcealed materials.

It is therefore desirable to have improved methods and systems foreffectively detecting high-Z materials, particularly accounting for thepossibility that such materials may be shielded by a combination ofhigh-Z materials for shielding gamma rays and low-Z neutron absorbersfor shielding neutrons.

SUMMARY OF THE INVENTION

In one embodiment, the application discloses an X-ray scanning systemfor identifying material composition of an object being scanned.

The aforementioned and other embodiments of the present shall bedescribed in greater depth in the drawings and detailed descriptionprovided below.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present specificationwill be further appreciated, as they become better understood byreference to the detailed description when considered in connection withthe accompanying drawings:

FIG. 1 illustrates normalized intensity of a transmitted X-ray beam perunit energy in relation to energy, in MeV, for different absorbers;

FIG. 2 illustrates a spectral ratio of the amplitudes of the low andhigh energy regions in the transmission spectrum shown in FIG. 1,plotted against atomic number;

FIG. 3 illustrates the X-ray transmission spectroscopy system scanning acargo container, in accordance with an embodiment of the presentinvention;

FIG. 4 illustrates the X-ray transmission spectroscopy system scanning acargo container, in accordance with another embodiment of the presentinvention; and

FIG. 5 illustrates an exemplary data flow for said X-ray transmissionspectroscopy system.

DESCRIPTION OF THE INVENTION

The present specification is an improved method of screening cargo thatuses spectroscopic information from only the transmitted, or from boththe transmitted and scattered, high-energy X-ray beam to provideenhanced detection capabilities of contraband, threats and other targetsof interest, which are difficult to detect with current X-ray methodsand/or through passive radiation detection techniques known in the art.The system of the present invention delivers improved detectionperformance for objects of interest either automatically or as a tool toassist an operator, while at the same time reducing false-alarm rate.

In general, for a given thickness of an absorber, the higher the atomicnumber, the higher the attenuation of the high end of the X-rayspectrum. Therefore, the transmitted X-ray spectrum is affected byvariations in the atomic number of items of various materials inside thecargo.

The present specification detects and measures the entire energyspectrum of the transmitted or transmitted and scattered X rays, andidentifies materials that are in the beam path and their probable atomicnumber and areal density. The energy spectrum of the X-rays transmittedthrough and scattered by a cargo contains a wealth of information on thematerial properties of the cargo they traverse. Theoretical analysis andactual measurements demonstrate that the X-ray spectrum of thetransmitted X-rays is very sensitive to the Z of the cargo materials.

The present invention is directed towards multiple embodiments. Thefollowing disclosure is provided in order to enable a person havingordinary skill in the art to practice the invention. Language used inthis specification should not be interpreted as a general disavowal ofany one specific embodiment or used to limit the claims beyond themeaning of the terms used therein. The general principles defined hereinmay be applied to other embodiments and applications without departingfrom the spirit and scope of the invention. Also, the terminology andphraseology used is for the purpose of describing exemplary embodimentsand should not be considered limiting. Thus, the present invention is tobe accorded the widest scope encompassing numerous alternatives,modifications and equivalents consistent with the principles andfeatures disclosed. For purpose of clarity, details relating totechnical material that is known in the technical fields related to theinvention have not been described in detail so as not to unnecessarilyobscure the present invention.

Referring to FIG. 1, the normalized intensity of the transmitted X-raybeam per unit energy 115 is shown in relation to the energy, in MeV, 105for different absorbers. As shown, for 9 MV X-ray transmission spectra,carbon (Z=6) has a very different spectrum from that of uranium (Z=92).Referring to FIG. 2, a spectral ratio 205 of the amplitudes of the lowand high energy regions in the transmission spectrum (shown in FIG. 1),plotted against atomic number 215, is shown, demonstrating a very highsensitivity 225.

Referring to FIG. 3, the X-ray transmission spectroscopy system 300 ofthe present invention is shown scanning a cargo container 305. Thesystem 300 employs a collimated X-ray source 335 and measures the energyspectra of the transmitted X-rays 315 using fast spectroscopic detectors325 with predetermined energy resolution. The higher the X-ray sourceenergy 335, the stronger the effect. In another embodiment, the system300 measures the energy spectrum of X-rays 315 scattered at a smallangle with respect to the original X-ray direction. It should beappreciated that the X-ray source can be of any energy level, but ispreferably an energy level of 1 MeV or higher. Further, the X-ray sourcecan be a pulsed source like an electron linac, a continuous X-rayemission source, an intensity-modulated X-Ray source, such as the onedisclosed in U.S. patent application Ser. No. 12/484,172 which isincorporated herein by reference, or any other type of X-ray source, andcan use any beam geometry, including pencil, fan, conical, or othergeometries.

Referring to FIG. 4, in another embodiment of the present invention, asystem 400 measures both the transmitted X-ray energy spectrum and thelarge-angle (>90°) scattered energy spectrum. Measuring both sets of thespectra simultaneously further enhances the Z sensitivity of system 400.In FIG. 4, the X-ray transmission spectroscopy system 400 is shownscanning a cargo container 405. The system 400 employs X-ray sources 435and measures the energy spectra of the transmitted X-rays 415 using fastspectroscopic detectors 425 with predetermined energy resolution. Thesystem 400 also measures the energy spectra of backscattered X-rays 465with respect to the original X-ray direction, as determined using X-rayshielding and collimation 445, using collimated high energybackscattered X-ray detectors 495. The energy spectra of some of theX-rays scattered at an angle greater than 90 degrees are measured inaddition to the transmitted energy spectra. Information from the twosets of spectra derived from X-rays 415, 465 can be combined to greatlyincrease the Z sensitivity. In this embodiment, collimated detectors 495are used which measure the energy spectra of the high-energyback-scattered X-ray radiation 465. By way of example, it may benecessary to move the cone of view up or down in order to scan an entireslice of the cargo illuminated by the X-ray fan beam. Other scanningapproaches are possible.

The proposed method can be used with pulsed X-ray sources, using, e.g.,electron linear accelerators (linacs), as well as CW (continuous wave)X-ray sources. Conventional linacs produce X-rays in short bursts ofradiation (usually less than 5 μs). In this case, the instantaneous rateof X-rays arriving at the detector during the pulse can be very high.This rate is especially high when no cargo is present (“in air”) and forlightly loaded containers where the X-ray transmission is high. If thecount rate is high enough, it is possible that the signals in the X-raydetector due to two or more X-rays overlap in time, in such a way thatthe energy of the individual X-rays cannot be reliably measured. Thiseffect is exacerbated if the X-ray detectors and their read-out systemsare not sufficiently fast. Even in this case, however, materialdiscrimination is still possible within the range of X-ray attenuationsby the cargo where the count rate does not exceed this threshold.Alternatively, a shield can be put in front of the detectors in order toreduce the count rate, but this may be done at the expense of being ableto perform spectroscopy at high attenuation.

CW sources produce X-rays continuously in time. For such sources, theinstantaneous count rate is lower than for pulsed sources with the same(integrated) output. This allows extending the applicability of thepresent method to a wider range of cargo attenuations.

In one embodiment of the present invention, a secondary array of veryfast detectors is used to obtain the spectral information, in additionto the primary detector array used for radiography. The first array ofdetectors, which generate a high-resolution radiographic image, may beless fast relative to the second array of detectors and need not measurespectra. In this manner, very fast detectors can be used fortransmission spectroscopy, e.g. plastic scintillators withphoto-multiplier tubes. In such a system, the spatial resolution of thespectroscopic system may not match that of the radiography system, forexample, because, if photomultiplier tubes are used, they tend to berelatively large compared to the photodiodes typically used forradiography. The spectroscopic array could be placed beside theradiographic array, or behind it. In this embodiment, the traditionalhigh-resolution radiographic image is maintained with a lower-resolutionspectroscopic image. This may provide a good trade-off betweencapability and cost.

In another embodiment of the system, for example in a mobile systemswith a much lower penetration requirement, one can use slower, butdenser, scintillator materials, such as LaBr₃ (Lanthanum Bromide) orLYSO (Lutetium Yttrium Ortho-Silicate). This allows one to make compacttransmission spectroscopy detectors with better spatial resolution. Inthis embodiment, the imaging and transmission spectroscopy arrays can becombined into a single array, since these scintillator materials arealso suitable for use in the primary imaging system.

Detector arrays that are only used for imaging usually employ slowscintillators, such as CsI (Cesium Iodide) or CdWO₄ (Cadmium Tungstate),with unbiased PIN photodiodes which are used in “integration” mode, i.e.they measure the total amount of energy deposited in them during anaccelerator pulse when a pulsed source is used, or during a fixed timeperiod when a CW source is used. Imaging detector arrays that are alsoused for transmission spectroscopy must use dense but fasterscintillators, such as the already mentioned LaBr3 or LYSO, and a fasterlight detector, such as a biased PIN diode. Alternate embodimentsinclude utilization of avalanche photodiodes and/or silicon driftdetectors. It should be appreciated, however, that any detector materialand read-out method can be used that is fast enough for the intendedpurposes as described herein. This includes anyscintillator/photo-detector combination, as well as any semiconductordevice suitable for detecting X-rays and measuring their energy, whichare fast enough for the intended purposes as described herein.

Combining the detector arrays accomplishes high-resolution radiographyand transmission spectroscopy with a single detector array. Adisadvantage is the possibly high cost of a large number ofspectroscopic channels. If high-resolution radiographic images are notrequired, large detectors can be used.

Regardless of the detectors used, the spectroscopic information isanalyzed using one of at least two analysis methods. In the first case,material separation is achieved employing various spectral features. TheX-ray transmission spectra are normalized by dividing each spectrum bythe measured total transmitted X-ray or energy flux. Using such anormalization method, the spectral shapes are unique for each materialZ. This approach provides a good separation of high-Z materials fromlower-Z materials, since the spectra have distinctively different peaklocations, intensities, widths, statistical skewness and other features.For example, the mean energies of spectra of high-Z materials are lower,and the peaks are narrower and have higher amplitudes. There is verygood separation between medium-Z (e.g. iron) and high-Z materials, andbetween medium-Z and hydrogenous materials. An example of the Zdependence of a spectral feature (in this case the ratio of theamplitudes of the lower and higher energy regions of the spectrum) isshown in the FIG. 2. There is a very strong correlation between thefeature selected here and the atomic number.

In an alternate analysis method, the spectrum for at least one detectoris fitted to an expected spectrum. The expected spectrum is computedfrom the incident spectrum of X-rays produced by the source bycalculating the attenuated X-ray spectrum through specific materials andcorrecting for the detector response. The areal densities of thecandidate materials are parameters of the fit. In this approach, aninitial material-composition estimate is computed based on the observedtransmission. With a list of constraints, including non-negativethickness, a least-squares (or other) statistical minimization isperformed until the difference between the computed and observed spectrais minimized. In some embodiments, the minimization is performed in twoor more steps. In the first step, a small number of material parametersare used to serve as an estimate, and in subsequent iterations anincreasing number of material parameters are used. In the finaliteration, all the materials in the considered set are used.

In both analysis methods, the results are given as combinations of arealdensities of materials that are likely to be present in the cargo, forexample, and not limited to such values or materials, 100 grams/cm² ofwood and 50 grams/cm² of steel.

In principle, employing detectors with sufficient energy resolution andwith high counting statistics, it is possible to determine the completeelemental composition of cargo along the beam path. In practice (withcurrent technology), materials with similar atomic numbers cannot easilybe distinguished. Typical detection groups include organic materials, orLow Z: (Z≦10), Medium-low Z: 11≦Z≦19, Medium Z: 20≦Z≦39, Medium-high Z:40≦Z≦72, and High-Z: (Z≧73). The number of Z-groups and their bounds maybe chosen in different ways, based on the energy of the X-ray generatingsource, whether it is pulsed or continuous, its intensity, the type andsize of the spectroscopic detectors, inspection time, etc.

The technology described above provides a system and methods to obtainenhanced material discrimination employing X-rays. The system andmethods of this invention improve the detection of contraband, threatsand other targets, allow easier cargo-manifest verification, andfacilitate automatic detection. These technical advantages translate toincreased operator accuracy and efficiency, leading to a reduction ofman power, increased contraband interdiction and increased customs-dutyrevenues.

The above examples are merely illustrative of the many applications ofthe system of the present invention. Although only a few embodiments ofthe present invention have been described herein, it should beunderstood that the present invention might be embodied in many otherspecific forms without departing from the spirit or scope of theinvention. Therefore, the present examples and embodiments are to beconsidered as illustrative and not restrictive, and the invention may bemodified within the scope of the appended claims.

We claim:
 1. An X-ray scanning system for identifying materialcomposition of an object being scanned comprising: at least one X-raysource for projecting an X-ray beam on the object, at least a portion ofthe projected X-ray beam being transmitted through the object; at leasttwo arrays of detectors, wherein one of said at least two arrays ofdetectors comprises scintillator material having at least one ofLanthanum Bromide or Lutetium Yttrium Ortho-Silicate, measures an energyspectra of the transmitted X-rays, and generates an image having a firstresolution, and wherein a second of said at least two arrays ofdetectors comprises scintillator material having at least one of CesiumIodide or Cadmium Tungstate and generates an image having a secondresolution, said second resolution being greater than the firstresolution; and a processor for identifying the material composition ofsaid object wherein said processor determines the material compositionusing said spectra.
 2. The X-ray scanning system of claim 1 wherein thesecond one of said at least two arrays of detectors detects saidtransmitted X-rays and generates a transmission image.
 3. The X-rayscanning system of claim 2 wherein the processor determines the materialcomposition using said spectra and said transmission image.
 4. The X-rayscanning system of claim 2 further comprising an array of collimatedbackscatter X-ray detectors for measuring energy spectra of X-raysscattered by the object at an angle greater than 90 degrees andgenerating backscatter data therefrom.
 5. The X-ray scanning system ofclaim 4 wherein the processor determines the material composition usingsaid spectra, said backscatter data, and said transmission image.
 6. TheX-ray scanning system of claim 1 wherein said identification of thematerial composition comprises at least one of determining an atomicnumber of at least some material in said object, determining an atomicnumber range of at least some material in said object, or determining anareal density of at least some material in said object.
 7. The X-rayscanning system of claim 1 wherein said processor determines thematerial composition using said spectra by: receiving said energyspectra; normalizing said energy spectra using a value; and determiningthe material composition of the object based upon said normalized energyspectra and a plurality of known spectra.
 8. The X-ray scanning systemof claim 7 wherein said normalized energy spectra is compared to theplurality of known spectra and wherein the material composition isidentified based upon said comparison.
 9. The X-ray scanning system asclaimed in claim 1, wherein the X-ray beam has a pencil shape, fanshape, or conical shape.
 10. The X-ray scanning system as claimed inclaim 1, wherein the X-ray source comprises at least one of a continuousX-ray source, pulsed X-ray source, intensity-modulated X-ray source,electron linear accelerator, or X-ray source having an energy level of 1MeV or greater.
 11. The X-ray scanning system of claim 1 wherein saidprocessor determines the material composition using said spectra by:receiving said energy spectra; fitting at least one of said energyspectra to an expected energy spectrum, wherein said expected energyspectrum is at least one of a plurality of previously measured X-rayspectra generated by transmitting and detecting X-rays through knownmaterials and correcting said detected X-rays to account for variationsin detectors; and identifying the material composition of the objectbased upon said fitting.
 12. The X-ray scanning system of claim 11wherein said processor generates a first estimate of said materialcomposition based on at least one of said spectra of the transmittedX-rays and not based on said fitting.
 13. The X-ray scanning system ofclaim 11 wherein said processor identifies the material composition ofthe object by minimizing differences between at least one of saidspectra of the transmitted X-rays and said expected energy spectrum. 14.The X-ray scanning system of claim 11 wherein each of said expectedenergy spectrum is specific to a particular material.